Inverter



Jan. 4, 1944. H. KLEMPERER v 2,338,118

INVERTER Filed Aug. 6, 1940 v 4 Sheets-Sheet 1 OUTPUT PRIMARY d GuRRcN-r22 Ad ra cunnem In L-G Cmcum M f f Fl 6. Z. 2$ f L-C'Cmcun' K+t2 .."ll/-v flow-Ac: Acaoss The:

Pt -V v|-r.|-10u'\ L .C Cmcurr INVEN Q 9,5 ACROSS HANS KLEHPERER TTMI: I

Jail. 4, 1944 KLEMPERER 2,338,118

INVERTER Filed Aug. 6, 1940 4 Sheets-Sheet 2 INVENTOR HANS KLEMPERER IBY a'fl XT-rm Jan. 4, 1944. H; KLEMPE-RER 2,338,118

INVERTER Filed Aug. 6, 1940 4 Sheets-Sheet 3 l3 32 l8 I v -4 0.05OURCEJhwzm-o R. HANs KLENPERER Patented Jan. 4, 1944 i INVERTER HansKlemperer, Belmont, Mass, assignor to Raytheon Manufacturing Company,Newton,

Mass, a corporation of Delaware Application August 6, 1940, Serlal No.351,630

18 Claims. This invention relates to inverters of the series typeutilizing controlled gas filled electrical discharge tubes forconverting direct current into alternating current.

Heretofore when it was attempted to increase the frequency or the powerhandled by such converters, failure has occurred due to the fact thatan'insuflicient de-ionization time was available for each tube andtherefore control of the tube was destroyed.

An object of this invention is to increase the reliability of operationof inverters of the foregoing type so that de-ionizati'on and consequentprotection of the system against short circuiting v the direct currentsource is reliably secured.

Another object of this invention is to increase both power and thefrequency which such inverters are capable of handling.

A further object is to devise an arrangement in which the forwardvoltage on each tube is suitable direct current source. Likewise,- aninductance iii. in parallel with a condenser H is connected from theanode 5 tea conductor l2 which extends to the positive side of thedirect current source. The anode 3 andthecathode 8 are connected to theends of the primary winding 13 on an output transformer l4 having asecondary winding l9 which is adapted to be connected to some suitablenu'tput device. A pair of condensers l5 and I6 are connected in seriesbetween the conductors 9 and I}. The primary winding 13 is provided witha center tap I! from whicha conductor I8 is connected to a pointintermediate the condensers l5 and IS.

The tubes l and 2 are provided with-suitable igniting electrode elementsand 2| for the. cathodes 4 and B respectively. Although these ignitersare of any suitable kind, they preferably increased when conduction isdesired but is-decreased when non-conduction is desired.

A still further object is to increase the inductance in series with thedirect current source so as to limit the rate of rise of short circuitdirect currents without, at the same time, substantially increasing theimpedance of the system to alternating currents of the output frequency.

The foregoing and other objects of this invention will be bestunderstoodfrom the following description of exempliflcations thereof,reference being had to the accompanying drawings wherein:

Fig. 1- is a diagrammatic representation of an inverter system embodyingmy invention;

Fig. 2 is a set of curves illustrating the mode .of operation of thearrangement shown in Fig. 1;

Figs. 3, 4, and 5are diagrams of modifications of the arrangement shownin Fig. 15

Fig. 6 is a diagram of an embodiment of m invention utilizing a largernumber of tubes and capable of greater power and higher frequencyoperation; and p Fig. 'I is a set of curves illustrating. the mode ofoperationof the arrangement shown in Fig. 6. The inverter illustrated inFig. 1 consists of a .pair of controlled discharge tubes 1 and 2. Theseare preferably gas filled tubes of the controlled ignition type. Tube Icontains an anode 3 and a cathode 4, preferably of the mercury pooltype. The tube 2 likewise contains an anode [and a cathode 6 whichpreferably'is similar to cathode 4; An inductance I in parallel with acondenser by a thin glass layer.

are of the electrostatic type as described and claimed in the co-pendingapplication of Percy- L. Spencer, Serial No. 303,963, filed November 13,r 1939. Such igniters are generally a conductor separated. and insulatedfrom the cathode. pool In order to supply igniting impulses totheigniters 20 and 2|, a source of alternating current 22 is connected tothe'primary winding 23 of a peaking transformer 24 having a saturableleg 25 which carries a pair of secondary windings 26 and 21 in whichpeaked igniting voltages are generated. A pair'of conductors 28 and 29connect the secondary winding 26' between the cathode l and itsassociated igniter 20. A'pair of conductors 30 and ill connect thesecondary winding 2'! between the cathode 8 and its associated igniter2i.

When the system described above is energized, the condensers l5 and I6are each charged to a voltage which is approximately half the voltage ofthe direct current source connected between the conductors 9 and I2.Thereupon the secondary winding 26, for example, supplies an ignitingimpulse to the igniter 20 which starts an are spot on the cathode 4 andpermits the tube l to conduct current. The condenser l5 having beencharged to a direct current voltage as described above makes the anode 8positive with respect to the cathode l and thereforecauses the tube l toconduct a pulse of current which discharges the condenser I5. Thedischarge circuit of the condenser l5 may not be critically damped-and.therefore its discharge current may tend to be oscillatory. Therefore,when the condenser l5 discharges through the tube I, its voltage isreversed and the magnitude of the reversed voltage is determined by thedegree to which energy is absorbed in the output circuit. Since thevoltage which is impressed upon the condenser I6 is the sum of thedirect current source of voltage and the voltage on the condenser I 5,it will be seen that during the discharge of the condenser I5, thecondenser I6 is charged to a somewhat higher potential than that of thedirect curren source.

The discharge current for the condenser I and the charging current forthe condenser I6 flows primarily through the condenser 8 due to the factthat the inductance 1' offers a relatively high impedance to suchcurrents. Therefore the condenser B likewise is charged to a voltagewhich makes its upper end positive and its lower end negative. It willbe seen likewise that the charge on the condenser I6 is such that itsleft end is negative and its right end is positive. The voltage on thecondenser I6 substantially neutralizes the voltage of the direct currentsource so as to tend to extinguish the tube I. In addition, the voltageon the condenser 8 is in a direction so as to extinguish the tube I. Theresult, therefore, is that when the condenser I5 is discharged thecondensers 8 and I6 extinguish the flow of current in the tube I. Theforegoing operation causes a short pulse of current toflow from thecenter tap I! through the left half of the primary winding I3. At somelater time, the secondary winding 21 supplies an igniting impulse to theigniter 2| which causes the tube 2 to fire. Current now flows in such adirection as to discharge the condenser I6 and charge the condenser I 5to a higher voltage than it initially had, in a manner similar to thatdescribed in connection with the initial discharge of condenser I5. Inthis case also, the discharge current for condenser I6 and the chargingcurrent for the.

condenser I5 flows primarily through the condenser II and the charge onthe condensers II and I5 extinguishes the discharge in the tube 2.Thisaction likewise causes a short pulse of current to flow to thecenter tap I! through the ht half of the primary winding I3.

The flowing of current pulses through each half of the primary windingI3 introduces an additional commutating action into the system. Whentube l, for example, is fired so as to produce a pulse of currentflowing from the center tap 11 through the left half of the prima yWinding I3, a back voltage is generated opposing such flow of current.This back voltage is likewise generated in the right half of the primarywinding I3 and is in the non-conducting direction of tube 2. If at thistime, there is any tendecy for tube 2 to conduct current in the forwarddirection, the voltage impulses thus generated cause such tendency to beextinguished.

In absence of the parallel circuits comprising the inductance I and thecondenser 8, and the inductance Ill and condenser II, shortly after thestarting of either tube I or 2, a relatively high forward voltage wouldbe impressed on the opposite tube. This would occur within a very shorttime interval after conduction in said opposite tube ceases. This timeinterval would be decreased as the frequency would increase. In thepresent invention, however, the charges on the condensers 8 and IIrespectively do not immediately disappear inasmuch as they can only bereversed through the inductances I and I0 respectively. Therefore, thevoltages on these condensers 8 and II remain as voltages in oppositionto the forward voltage applied to the associated tubes following thestarting of theop posite tube. The effect, therefore, is to delay theapplication of the forward voltage to the tubes I and 2 and thus givethem alonger time in which to de-ionize. The frequency of the parallelinductance and capacity circuits is not critical, but the maximum effectis produced when the frequency of each of these circuits equals thefrequency of firing of the associated tube. Under these conditions, whenthe time arises for either tube I or 2 to fire, the charge on theassociated condensers 8 or II will have reversed and thus be in adirection to assist the tube to fire. Part of the energy which is storedin the charge on the condenser will, under these conditions, bedelivered as useful energy to the out put system by the current flowingthrough the primary winding l3 and due to the fact that the dischargecircuit for such condensers contains considerable inductance, part ofthe energy will be returned-to the condenser to charge it in theopposite direction so as to assist in the de-ionization of itsassociated tube.

The foregoing operation may be more clearly understood from the curvesof Fig. 2. Along axis a are shown a series of current pulses drepresenting the current pulses delivered to the primary winding I3. Theleft hand current pulse may, for example, be delivered as a result ofthe firing of tube I. A substantially similar pulse of current J shownon the b axis is delivered to the condenser 8. Said condenser, prior tothis time, has acquired a certain charge a will be pointed out below.When the pulse of current f is delivered to the condenser, itneutralizes the previous charge and causes the condenser to be chargedup in the opposite direction to a somewhat higher potential. Along theaxis a, the time during which the left pulse of current d flows isrepresented as k. During this time, the tube is conducting and itsvoltage, represented by g remains substantially constant at the voltagedrop of said tube. At the end of the time k the current 11 which islikewise flowing through the tube, tends to reverse and since the tubecannot conduct such reversed current, it stops conducting. In absence ofthe inductance I and condenser 8, when tube i stops conducting current,the reversed voltage of the condenser I5 would appear across it. Thisvoltage is represented by the lower horizontal portion of the dottedcurve It on the axis 0. In the arrangement as shown in Fig. 1, however,in addition to the reversed voltage of the condenser I5, the voltage onthe condenser 8 is likewise impressed on the tube I in thenon-conducting direction, so that the voltage which appears across thetube I in the non-conducting direction at the end of the time k is thesum of these two voltages as stated above. Since the back voltage isincreased to this extent, the ions are swept out of the discharge spaceat a more rapid rate and de-ionization is accelerated.

After the time k the condenser 8 starts to discharge through theinductance I at a rate determined by the natural frequency of theparallel inductance-capacity circuit as shown on the axis 2) in Fig. 2.In the arrangement as shown in Fig. 1, the natural frequency of theparallel inductance-capacity circuit is somewhat lower than the outputfrequency of the system. Due to the discharge current flowing from thecondenser 8, its voltage will decrease along the curve e, reverse indirection, and acquire a certain value in the opposite direction. Whenthe associated tube is again fired, the voltage which would appearcapacity circut represented by thedotted curve h after the time k,continues at the value of the reversedvoltage on the condenser II foratime h when the tube 2 is fired. Due to theback voltage generated inthe primary winding I! as pointed out above, an increased inversevoltage would be impressed on the tube causing the lower peak to occuron the curve 7;. Due to the firing of the tube 2 the next pulseoi'current d is supplied to the primary winding it. During this time, underthe conditions of operation, in absence of the parallel.inductance-capacity circuit, the discharge and revers l of voltage ofthe condenser l6 would cause the voltage across the tube l to decrease,reverse, and increase in the opposite direction as shown by the dottedcurve h. Since each tube is not extinguished exactly at the zero valueof current but at a value slightly above it, a back voltage generated inthe primary winding l3 upon the termination of the current how in thetube 2 causes a slight decrease in the value to which the condenser I5is charged dur ing the discharge and reversal of voltage on thecondenser i6. This produce the drop in the curve h from its upper peakvalue as shown in Fig. 2. Thereupon the voltage across the tubel wouldconduct along the upperhorizontal value of the curve h which likewiserepresents the value of the voltage acquired by the condenser l5.

" Since, as pointed out above, the resultant voltage on the tube l inthe arrangement shown in Fig. 1 is the summation of the voltage acrossthe inductance-capacity circuit and the voltage which would appearacross the tube in absence of said circuit, this resultant voltage maybe represented in Fig. 2 by the summation of the curves e and h. ---InFig. 2, curve g after the time k represents this summation. It will beseen by the curves on the axis that without the inductancecapacit-y crcuit, the voltage 'across tube 1 remains in the non-conductingdirection following the stoppageof conduction in said tube forthe timeis. However, due to the delay in the reversal of this voltage by theinductance-capacity circuit. in the present arrangement, the voltagethus remains in the non-conducting direction for a longer time t3. Thetime during which the voltage remains in the non-conducting direction isthe de-ionization time of the tube, and therefore, it will be seen thatby the present arrangement the de-ionization time of each tube can bedecreased. Furthermore, it will be noted that when the voltage on thetube I does reverse, it does not increase at as rapid rate as that whichwould occur in absence of the parallel inductancecapacity circuit. Sincethe tendency of this type of tube to break down before beingsuppliedwith an igniting impulse depends to a large degree on the rapidity atwhich the forward voltage 'is applied thereto, this decrease in the rateat which the forward voltage is applied increases the reliability ofoperation and decreases the tendency the condenser 8 a well as that ofthe condenser -stantially independent commutating means present in thearrangement shown in Fig. 1."

These are the two condensers Ill and I8, the'primary winding i3 actingthrough its back E. M. I". on each of the tubes, and the parallelinductancecapacity circuits associated with the two tubes. This makesfor a very reliable type of operation.

In some instances, however, 'one of the foregoingcommutating means maybe dispensed with and satisfactory operation obtained. For example, Fig.3 shows an arrangement in which the commutating effect of the centertapped primary winding l3 of Fig. l is eliminated by placing saidprimary winding directly in series with the conductor l8 extending to apoint intermediate the condensers l and it. In Fig. 3 where the elementsare identical with those hown in Fig. 1,

the same reference numerals are applied. Fig. 3

operates in the same way as described in connection with Fig. 1- exceptthat the commutating effect of the center tapped primary winding 03 ofFig. l is eliminated. It will be noted that in Fig. 3, instead of takingthe output energy from the upper connection between the two tubes asshown in Fig. 1, the output energy is taken from the central connection.Under these conditions, instead of the pulses of output current beingdelivered successively in the same direction as in the case of Fig. 1,such pulses are delivered successively in opposite directions in Fig. 3.

Fig. 4 shows an additional modification of Fig. l in which thecommutating effects of the condensers l 5 and I6 are eliminated. In Fig.4 likewise, where the elements are identical with those of Fig. 1 thesame reference numerals are applied. In Fig. 4 the series connection ofthe two condensers l5 and I6 of Fig. 1 is replaced by a voltage dividingresistance IS. The arrangement s own in Fig. 4- operate's in a mannersimilar to that described in connection with Fig. 1 'except that thecommutating effects of condensers l5 and i6 are eliminated and theenergy for commutating each tube is stored primarily in the condensers 8and II, respectively.

If it is desired to take oi the output energy from the centralconnection as shown in Fig. 3 and at the same time utilize thecommutating effects of a center tapped'co-il connected between the twotubes, the arrangement shown in Fig. 5 may be utilized. In this figure avoltage dividing resistance or inductance I5 is used as in Fig. 4instead of the condensers l5 and it of Fig. 1. The primary winding I3 isarranged in series with the central conductor It as in Fig. 3. In

- addition, an induction coil I3 is connected between the anode 3 andthe cathode 6. This coil is wound upon a suitable magnetic core [4. Thecenter tap H in this instance is on the coil l3. In Fig. 5 the systemoperates in a manner similar to that of Fig. 4 differing therefrom inthe output current impulses in the same way as Fig. 3 differs from Fig.1.

If it is desired to increase either the power or the frequency of theinverter over that which is practical for the foregoing embodiments, asystem utilizing a larger number of tubes may be 15 so that the forwardvoltage on said tube is used. Such an arrangement is shown in Fig.6.Here also where the elements are identical with those in Fig. 1, thesame reference numerals are applied. Instead of using two tubes as inFig. 1,

however, a plurality of pairs of tubes 32, 33, 34,

35, 36, and 31 are provided. Each of these tubes is'preferably' similarto the tubes I and 2 of Fig. 1 and are therefore provided with an anode,pool type cathode, and an igniting electrode element.

The tubes 32, 33, and 34 have their anodes connected to one side of theprimary winding I3 while the tubes 35, 36, and 31 have their cathodesconnected to the right end of said primary winding in a manner similarto that described in connection with tubes l and 2 of Fig. 1.- In Fig.6, each of the tubes has connected in series there with a parallelinductance-capacity circuit consisting of an inductance 38 and acapacitor 39. Each of the igniting electrode, elements is supplied withigniting impulses from an igniting transformer 40, having a primarywinding 4! preferably connected in delta. Said primary winding isconnected through leads 42, 43 and 44 to a suitable source of threephase alternating current 45. The igniting transformer 40 is providedwith a six phase secondary winding 46 connected in star. The secondarywinding 46 consists of secondary coils 41 to 52 inclusive. The neutralpoint of the secondary winding 46. is provided with a ground connection53. In series between the outer ends of the secondary coils ill to 52inclusive and ground, are connected primary windings 54 to 59 inclusiveof peaking transformers 60 to 65 inclusive. These peaking transformersare provided with secondary windings 66 to H inclusive. Connections aremade from the peaking transformers to the tubes so that the tubes onopposite sides of the inverters are alternately fired in sequence. Forthis purpose, the secondary windings of the peaking transformers areconnected to the tubes as follows: secondary winding 66 to tube 32,secondary winding 61 to tube 35, secondary winding 68 to tube 33,secondary winding 69 to tube 36,- secondary winding 10 to tube 34, andsecondary winding 1| to tube 31. Each igniter of the type describedabove produces an arc spot on its associated cathode when it is suppliedwith a voltage impulse which is positive with respect to the cathode.The peaking transformers, however, each supplies pulses which arealternately opposite in polarity. As shown in Fig. 6, by reversing theconnections to the secondary windings 69 to H, each compared to thesecondary windings 66 to 68, the alternate firing of tubes on oppositesides of the inverter may be produced.

The operation of the arrangement shown in Fig. 6 may be more clearlyunderstood by referring to the curves of Fig. '7. Along axis a are showna series of current pulses d representing pulses of current delivered tothe primary winding I 3. The left hand current pulses may, for example,be delivered as a result of the firing of tube 32. As described inconnection with Fig. 2, during this period which is again represented byk a pulse of current f is delivered to the associated condenser 39,causing the voltage e of said condenser to reverse as indicated on theaxis b of Fig. '7. Thereafter, the charge on the condenser 39 dischargesthrough its associated inductance 38 in a pulsatory manner so that thevoltage e of said condenser reverses, also as shown on the axis b, whilethe current in the inductance-capacity circuit varies in accordance withthe curve f. In absence of the use of the condenser 39 and inductance38, the voltage across each tube in the arrangement of Fig. 6 wouldfollow the dotted curve h on the axis of Fig. '7. However in thearrangement shown, the voltage e of the inductance-capacity circuit isadded to said voltage h and the resultant voltage across each tube isrepresented by the summation curve a. From Fig. 7, it will be seen thatin absence of the inductance 38 and capacity 39,

the de-lonlzation of each tube would be represented by the relativelyshort time is while in the arrangement of Fig. 6, the de-ionization isconsiderably increased to the value as represented by 153. In order toproduce the maximum effect of the parallel inductance-capacity circuit,the frequency of oscillation is selected as a sub-multiple of the outputfrequency so that the voltage across each of the condensers 39completely reverses between each pulse of charging current suppliedthereto. As previously stated, the frequency-of this inductance-capacitycircuit is not critical and may vary considerably from such a Value.

Of course, it is to be understood that this inventionis not limited tothe particular details as described above as many equivalents willsuggest themselves to those skilled in the art. For

example, other types of controlled discharge tubes may be utilized aswell as various other numbers of such tubes. Other variations andutilizations of principles innumerable will suggest themselves to thoseskilled in the art. It *is accordingly desired that the appended claimsbe given a broad interpretation commensurate with the scope of theinvention within the art.

-What is claimed is:

1. An inverter comprising a space discharge tube, a commutating meansand an output means in series, means for connecting said inverter to asource of voltage, and means for generating a periodic voltage tendingto increase current flow throughsaid tube during the conducting periodand to oppose such flow of current during the non-conducting periodfollowing stoppage of current flow upon commutation.

2. An inverter comprising a space discharge tube, a commutating meansand an output means in series, means for connecting said inverter to asource of voltage and means for generating a periodic voltage aiding thefirst named voltage during flow of current through said tube andopposing said first named voltage upon stoppage of current flow by saidcommutating means.

3. An inverter comprising a space discharge tube, a commutating meansand an output means in series, means for connecting said inverter to asource of voltage, and means responsive to the initiation of currentthrough said tube to increase the magnitude of said current and todecrease the 'tendency of said current to flow during the nonconductingperiod following stoppage of current by said commutating means.

4. An inverter comprising a space discharge tube, a commutating means,an impedance means, and an output means in series, and means forconnecting said inverter to a source of voltage, said impedance meanshaving a relatively high inductance value and a relatively lowalternating current impedance value to periodic currents ofsubstantially the output frequency of said inverter.

5. An inverter comprising a. space discharge tube, a commutating means,an impedance means, and an output means in series, and means forconnecting said inverter to a source of voltage, said inverter beingtuned to substantially the desired output frequency, said impedancemeans having a relatively high inductance value and such a relativelylow alternating current impedance value that said tuned frequency issubstantially unimpeded by said impedance means.

6. An inverter comprising a space discharge tube, a condenser, and anoutput means in series, an inductance and a condenser connected inparallel with each other in series with said tube, and means forconnecting said inverter to a source of voltage. I

7. An inverter comprising a space discharge tube, a condenser, and anoutput means in series,

an inductance and a condenser connected in parallel with each other inseries with said tube,

. being tuned to a substantially'lower frequency than said outputfrequency. v

9. An inverter comprising a space discharge tube, a condenser, and anoutput means in series, and means for connecting said inverter to asource of voltage, said inverter having a predetermined outputfrequency, an inductance and a condenser connected in parallel with eachother in series with said tube, said parallel circuit being tuned to asubstantially lower frequency than said output frequency.

10. An inverter comprising a space discharge tube, a condenser and anoutput means in series, means for connectingsaid inverter to a source ofvoltage, said inverter having a predetermined output frequency, aninductance and a condenser connected in parallel with each other inseries with said tube, said parallel circuit being tuned to asubstantially lower frequency than said output frequency, and means fordischarging said first-named condenser.

11. An inverter comprising a plurality of space discharge tube paths, acommutating means and an output means in series and a separateindependent inductance and a condenser connected in parallel with eachother in series with each of said paths, and means for connecting saidinverter to a source of voltage.

12. An inverter comprising a plurality of space discharge tubesconnected in paralleland connected in series with a commutating means,and feeding a common output means, means for connecting said inverter toa source of voltage, an independent inductance and a condenser connectedin parallel with each other in series with condenser connected inparallelwith each other in series with each of said tubes and means forfiring--v each of said tubes at a submultiple of the desired'gutputfrequency, said firing means being displaced in phase so that said tubessupply pulses of current successively to said output means..

14. An inverter comprising two condensers connected in series andadapted to be connected across a source of current, a space dischargetube feeding an output means and connected between one end of saidseries circuit and a point between each of said tubes and means forfiring each of said tubes at a submultiple of the desired outputfrequency, said firing means being displaced in phase so that said tubessupply pulses of current successively to said output means.

13. An inverter comprising a plurality of space discharge tubes eachconnected in series with a commutating means and feeding a common outputmeans, an independent inductance and a said condensera'another spacedischarge tube feeding said output' means and connected between theother end of said series circuit and said point, an independentinductance and a condenser connected in parallel with each other inseries with each of said tubes, and means for alternately firing saidtubes.

15. An inverter comprising two condensers connected in series andadapted to be connected across a source of current, a space dischargetube feeding an inductance and connected between one end of said seriescircuit and a point between said condensers, another space dischargetube feeding said inductance, and .connected between the other endofsaid series circuit and said point, an independent inductance and acondenser connected in parallel with each other in series with each ofsaid tubes, and means for alternately firing said tubes.

16. An inverter comprising two condensers connected in series andadapted to be connected across a source of current, a space dischargetube feeding a part of an inductance and connected between one end ofsaid series circuit and a point between said condensers, another spacedischarge tube feeding another part of said. inductance and connectedbetween the other end of said series circuit and said point, anindependent inductance and a condenser connected in parallel with eachother in series with each of said tubes, and means for alternatelyvfiring said tubes.

17. An inverter comprising a resistance adapted to be connected across asource of current, a space discharge tube feeding an output means andconnected between one end of said resistance and an intermediate pointin said resistance, another space discharge tube feeding said outputmeans and connected between the other end of said resistance and saidpoint and means for alternately firing said tubes.

18. An inverter comprising a space discharge tube, a commutating means,an impedance means, and an output means in series, and means forconnecting said inverter to a source of voltage,

said impedance meanshaving a relatively high

